Acid rain
Acid rain occurs when
sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in
clouds. The droplets then fall to earth as
rain,
snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.erm "acid rain" is sometimes used more generally to include all forms of acid deposition - both wet deposition, where acidic gases and particles are removed by rain or other precipitation, and dry deposition removal of gases and particles to the Earth's surface in the absence of precipitation.rain accelerates
weathering in carbonate rocks and accelerates
building weathering.
Encyclopedia
Acid rain occurs when
sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in
clouds. The droplets then fall to earth as
rain,
snow, or sleet. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.
The term "acid rain" is sometimes used more generally to include all forms of acid deposition - both wet deposition, where acidic gases and particles are removed by rain or other precipitation, and dry deposition removal of gases and particles to the Earth's surface in the absence of precipitation.
Acid rain is defined as any type of precipitation with a
pH that is unusually low. Dissolved carbon dioxide dissociates to form weak
carbonic acid giving a pH of approximately 5.6 at typical atmospheric concentrations of CO
2. Therefore a pH of <5.6 has sometimes been used as a definition of acid rain. However, natural sources of acidity mean that in remote areas, rain has a pH which is between 4.5 and 5.6 with an average value of 5.0 and so rain with a pH <5 is a more appropriate definition.
The US EPA says, "Acid rain is a serious environmental problem that affects large parts of the US and Canada."
Acid rain accelerates
weathering in carbonate rocks and accelerates
building weathering. It also contributes to acidification of
rivers, streams, and
forest damage at high elevations.
History and trends
Evidence for an increase in the levels of acid rain comes from analysing layers of glacial ice. These show a sudden decrease in pH from the start of the industrial revolution of 6 to 4.5 or 4. Other information has been gathered from studying organisms known as
diatoms which inhabit ponds. Over the years these die and are deposited in layers of
sediment on the bottoms of the ponds. Diatoms thrive in certain pHs, so the numbers of diatoms found in layers of increasing depth give an indication of the change in pH over the years.
Since the industrial revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased. Occasional pH readings of well below 2.4 have been reported in industrialized areas. Industrial acid rain is a substantial problem in
China,
Eastern Europe,
Russia and areas down-wind from them. These areas all burn sulfur-containing coal to generate heat and electricity. The problem of acid rain not only has increased with population and industrial growth, but has become more widespread. The use of tall smokestacks to reduce local
pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. Often deposition occurs a considerable distance downwind of the emissions, with mountainous regions tending to receive the most . An example of this effect is the low pH of rain which falls in
Scandinavia.
Acid rain was first reported in
Manchester,
England, which was an important city during the
Industrial Revolution. In 1852, Robert Angus Smith found the relationship between acid rain and atmospheric pollution. The term "acid rain" was used for the first time by him in 1872issions of chemicals leading to acidification
The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides which are oxidised to form Nitric acid are of increasing importance due to stricter controls on emissions of sulfur containing compounds. 70 Tg per year in the form of SO2 comes from fossil fuel combustion and industry, 2.8 Tg from wildfires and 7-8 Tg per year from volcanoes.
Natural emissions
The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and those from biological processes that occur on the land, in wetlands, and in the oceans. The major biological source of sulfur containing compounds is Dimethyl sulfide.
The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe.
Human emissions
The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation, factories and motor vehicles. The gases can be carried hundreds of miles in the atmosphere before they are converted to acids and deposited.
Gas phase chemistry
In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via a termolecular reaction:
- SO2 + OH· + M? HOSO2· + M
which is followed by:
- HOSO2· + O2 ? HO2· + SO3
In the presence of water sulfur trioxide is converted rapidly to sulfuric acid:
- SO3 + H2O + M ? H2SO4 + M
Nitric acid is formed by the reaction of OH with Nitrogen dioxide:
- NO2 + OH· + M ? HNO3 + M
For more information see Seinfeld and Pandis .hemistry in cloud droplets
When clouds are present the loss rate of SO2 is faster than can be explained by gas phase chemistry alone. This is due to reactions in the liquid water droplets
Hydrolysis
Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:
- SO2 + H2O SO2·H2O
- SO2·H2O H++HSO3-
- HSO3- H++SO32-
Oxidation
There are a large number of aqueous reactions of sulfur which oxidise it from S to S leading to the formation of sulfuric acid. The most important oxidation reactions are with ozone, hydrogen peroxide and oxygen .
For more information see Seinfeld and Pandis .rosol formation
In the gas phase sulfuric and nitric can condense on existing
aerosols or nucleate to form new aerosols. The nucleation process is an important source of new particles in the atmosphere and so emissions of sulfur containing compounds, as well as causing acidification also have a climate effect.
Acid deposition
Wet deposition
Wet deposition of acids occurs when any form of precipitation removes acids from the atmosphere and delivers it to the Earth's surface. This can result from the deposition of acids produced in the raindrops or by the precipitation removing the acids either in clouds or below clouds. Wet removal of both gases and aerosol are both of importance for wet deposition.
Dry deposition
Acid deposition also occurs via dry deposition in the absence of precipitation. This can be responsible for as much as 20 to 60% of total acid deposition.
Soils
Soil biology can be seriously damaged by acid rain. Some tropical microbes can quickly consume acids but other types of microbe are unable to tolerate low pHs and are killed. The enzymes of these microbes are denatured by the acid. The hydronium ions of acid rain also mobilize toxins and leach away essential nutrients and minerals .
Forests and other vegetation
Acid rain can slow the growth of forests, cause leaves and needles to turn brown and fall off and die. In extreme cases trees or whole areas of forest can die. The death of trees is not usually a direct result of acid rain, often it weakens trees and makes them more susceptible to other threats. Damage to soils can also cause problems. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acid than rain.
Prevention methods
Technical solutions
In the United States, many coal-burning power plants use Flue gas desulfurization to remove sulfur-containing gases from their stack gases. An example of FGD is the wet scrubber which is commonly used in the U.S. and many other countries. A wet scrubber is basically a reaction tower equipped with a fan that extracts hot smoky stack gases from a power plant into the tower. Lime or limestone in slurry form is also injected into the tower to mix with the stack gases and combine with the sulfur dioxide present. The calcium carbonate of the limestone produces pH-neutral calcium sulfate that is physically removed from the scrubber. That is, the scrubber turns sulfur pollution into industrial sulfates.
In some areas the sulfates are sold to chemical companies as gypsum when the purity of calcium sulfate is high. In others, they are placed in a land-fill.
However, the effects of acid rain can last for generations, as the effects of ph level change can stimulate the continued leaching of undesirable chemicals into otherwise pristine water sources, killing off vulnerable insect and fish species and blocking efforts to restore native life.
International treaties
A number of international treaties on the long range transport of atmospheric pollutants have been agreed e.g. Sulphur Emissions Reduction Protocol and Convention on Long-Range Transboundary Air Pollution.
Emissions trading
An even more benign regulatory scheme involves emission trading. In this scheme, every current polluting facility is given an emissions license that becomes part of capital equipment. Operators can then install pollution control equipment, and sell parts of their emissions licenses. The main effect of this is to give operators real economic incentives to install pollution controls. Since public interest groups can retire the licenses by purchasing them, the net result is a continuously decreasing and more diffused set of pollution sources. At the same time, no particular operator is ever forced to spend money without a return of value from commercial sale of assets.
References
Further reading
- John McCormick, Acid Earth: The Global Threat of Acid Pollution .
External links
- - a 98-page report to Congress
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- U.S. Environmental Protection Agency -
- U.S. Geological Survey -
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See also
- List of environment topics
